2nd Gen Ram Tech1994-2001 Rams: This section is for TECHNICAL discussions only, that involve the 1994 through 2001 Rams. For any non-tech discussions, please direct your attention to the "General discussion/NON-tech" sub sections.
I'll give you some general info not specific to the Ram. Start by checking the coolant level. If thats good, check the thermostat. You could do that by starting your truck while the engine is cold and touching the upper (I think) radiator hose. It should stay cold until your engine reaches operating temp and then the hose should rapidly get hot. That signifies that your thermostat has opened. It still could be your thermostat since it opens and closes based on the temperature of the coolant and may be getting stuck wide open. To be honest, if you haven't changed it in years, go ahead and change it. If the coolant is fine, and the thermostat is good, I'd check for a blockage in the heater core.
I had the same problem it ended up being my radiator was clogged only the top would get hot and the bottom was ice cold feel your bottom hose on the radiator. So I replaced the thermostat and radiator and now the heat blows nice and hot.
Vehicle: 2007 Ram 1500 4x2 Big Horn aka The Silver Bullet
Location: Norman, Okla-mobile-homa
Make sure your windows are rolled up
Rolling coal is like spending a considerable amount of money to reconfigure your plumbing system so that when you shat in the toilet, it gets sprayed all over your furniture because someone suggested you should use a better-performing furniture polish.